This invention pertains to digital encoders of the type utilizing an opaque rotating disc provided with transparent sections the relative location of which provides the desired encoding.
One machine commonly used for decorating or merely coating cans and similar containers is the socalled continuous motion decorator to which a continuous stream of cans to be decorated is fed. A large wheel, rotating in its own plane, is provided with a plurality of mandrels mounted perpendicular to the wheel around its circumference. Each mandrel is mounted by one end in such a way as to be able to move towards the center of the wheel. Each can to be decorated is mounted on a mandrel, and rotation of the wheel brings each can in turn into contact with a printing roller, which prints the desired pattern on the exterior of the can, after which the rotation of the wheel withdraws the can from contact with the printing roller. This type of decorator is disclosed in U.S. Pat. No. 4,140,053 issued Feb. 20, 1979 to J. P. Skrypek, et al for a Mandrel Mounting and Trip Mechanism for Continuous Motion Decorator.
Sometimes a can is placed on a mandrel improperly or no can is placed on the mandrel at all. An improperly mounted can is removed from the mandrel by a blast of air and any mandrel without a can mounted on it is then moved toward the center of the wheel in order to prevent contact between the mandrel and the printing roller. This is necessary because any ink or varnish applied to a mandrel would be applied to the interior of the next can mounted on that mandrel, ruining that can and possibly causing it to stick to the mandrel. In the latter case it would be necessary to stop the machine in order to remove the can and to clean the mandrel.
To determine when a mandrel which is about to enter the decorating zone is improperly loaded or unloaded, a metal detector or other convenient detection means is located adjacent to the wheel. A timing device, commonly a digital encoder, is employed to activate the metal detector whenever a mandrel is expected to be passing the detector. Such an encoder typically comprises an opaque disc provided with a plurality of evenly spaced transparent openings about its circumference. The disc is rotated about its axis at a uniform speed such that one of the transparent openings passes a photocell each time one of the mandrels passes the detector. The photocell and its light source are placed on opposite sides of the encoder disc in such a way that when one of the openings in the disc passes between them, the light source illuminates the photocell, actuating the latter causing the associated metal detector to be actuated so that it then "looks for" a properly mounted can on the mandrel which is passing the detector at this time.
In order for the above-described arrangement to work properly, it is essential that one of the openings in the encoder disc pass between the photocell and the light source at exactly the instant when a mandrel passes the metal detector. If this coordination is lacking, it is generally necessary to stop the can decorator altogether and readjust the encoder for proper synchronization. This prior art procedure is no more than trial and error so that very often many intermediate adjustments must be made in order to obtain a prcise setting. Because of the great size of the decorator, this procedure is extremely cumbersome and inconvenient.